Cassidy attended York High School, in York, Maine. He then graduated from the Naval Academy Preparatory School in Newport, Rhode Island, in 1989. He received a Bachelor of Science degree in Mathematics from the United States Naval Academy in 1993 and a Master of Science degree in Ocean Engineering from MIT in 2000. While in the Navy, Cassidy passed BUD/S and became a Navy SEAL. While a SEAL he served several tours of duty supporting the fight in the War on Terror. His first spaceflight was on Space Shuttle mission STS-127, and his second was as a flight engineer for Expedition 35/36, launched aboard Soyuz TMA-08M. He was in space between July 15–31, 2009 and March 28 – September 10, 2013. Cassidy has worked as a CAPCOM for both International Space Station and Space Shuttle missions in the past.